Many measurement techniques produce data representing the variation of some physical property over a three-dimensional volume. For example, magnetic imaging techniques used in medical imaging generate data representing the density and environment of hydrogen nuclei at various locations in the subject's body. Similarly, CAT scans measure the x-ray absorbance at various locations in the subject's body. All of these techniques generate three- dimensional arrays of data in which each point in the array represents the measured physical property at a corresponding point in the object under examination.
Presenting the large amount of data generated by such imaging techniques in a manner that is easily assimilated by human observers presents significant computational problems. The process of converting three-dimensional arrays of data to images representing that data is referred to as volume rendering. Several approaches to this problem have been implemented. In general, these approaches assign "optical" properties to each point in a three-dimensional volume which is broken into small volumes referred to as voxels. Ideally, an image is then produced by calculating the image one would see on a screen located between the object and the viewer if a three-dimensional object having these optical properties were viewed from a given location when the object is illuminated. Unfortunately, the computational difficulty inherent in taking into account reflections, absorption, and scattering in each voxel are too great to allow an exact computation to be made. Hence, various approximations to the exact computation are utilized.
One general approach which is computationally manageable is referred to as ray casting. This approach assigns color and opacity values to each voxel. A ray is generated corresponding to each pixel on the viewing screen. At the ray passes through the various voxels, color values are integrated along the ray path. The resultant calculated value is essentially equivalent to the light reaching the screen along the ray that would be generated if each voxel consisted of a material which generated light of the color in question and absorbed light according to the assigned opacity value. High opacity values at a particular voxel will cause a strong color contribution from that voxel as well as attenuating color values generated by voxels behind the voxel in question. The contributions of secondary rays produced by reflections and refractions in other voxels are ignored in this approximation.
While this type of volume rendering generates useful images, the computational workload is prohibitive if the three-dimensional data array is large. As the resolution of the measuring devices increases, the size of the three-dimensional data arrays in question increase dramatically. An improvement of a factor of two in resolution results in a factor of eight increase in the size of the three-dimensional data array and a similar increase in the computational workload.
On conventional sequential computers, the time to calculate a single view of the data may be several hours. Such long computational times severely limit the usefulness of such imaging techniques. In general, the viewer must examine several such images to find the view point that best displays the features of the data which are of interest. Hence, interactive systems in which the user can move his or her viewpoint relative to the "object" are advantageous. To provide such interactive displays, parallel computer computation is necessary. Thus such systems are limited to computers that are not generally available to the average user.
Broadly, it is an object of the present invention to provide an improved imaging method and apparatus for visualizing three-dimensional data.
It is a further object of the present invention to provide an imaging method and apparatus which does not require special purpose computers.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the present invention and the accompanying drawings.